The surface to be hardened is strongly heated under the hardening device by the inductor subjected to alternating current. Arranged downstream of the inductor in the direction of treatment is a spray, from which a cooling fluid is applied to the previously heated-up surface for quenching. With the materials that are usually used, an austenite that is as homogeneous as possible is produced in a surface layer by the heating, as high a proportion as possible of martensite then being formed by the quenching at a high cooling rate. The effective depth of hardening depends in this case on the temperature distribution in the surface layer and can be changed during the induction hardening, for example by means of the frequency, which may particularly lie between 3 kHz and 8 kHz, and the power output.
In order to harden the closed curve trace, for example a running surface of a bearing ring, the curve trace may be moved continuously past a fixed hardening device. Equally, however, the workpiece may remain at rest, while the hardening device is moved along the surface to be hardened. In principle, there is also the possibility of clamping the workpiece on a support.
In the case of bearing rings for large rolling bearings, a method in which the bearing ring is moved past a fixed hardening device has proven successful. In particular, the bearing ring may be arranged vertically or at an angle, the movement of the bearing ring then being able to take place by means of supporting rollers.
In principle, there is the problem that already hardened regions must not be heated again to a temperature at which the hardness properties are lost again. In the case of a closed curve trace, for example a running surface of a bearing ring, an unhardened slip zone is therefore provided between an initial zone, at which the hardening is begun, and an end zone, which is hardened last. The unhardened slip zone typically has a width of between 10 mm and 20 mm, it being known in the case of rolling bearings to relief-grind the slip zone, in order that the rolling bodies do not have bearing contact there.